1. Field of the Invention
The present invention relates to an apparatus for aligning a substrate to a desired position in a semiconductor manufacturing apparatus, and more particularly to an alignment apparatus of an exposure apparatus, for a substrate to which the image is to be transferred.
2. Related Background Art
In semiconductor manufacturing apparatus (exposure apparatus, repair apparatus, inspection apparatus etc.), it has conventionally been required to align a semiconductor wafer on which circuit chips are prepared, with a high precision, in said apparatus.
Particularly in the exposure apparatus, it is necessary to precisely detect, in advance, the position of the circuit pattern on the water, in order to precisely align the circuit pattern of a reticle or a mask with that on the wafer.
In a reduction projection exposure apparatus of step-and-repeat method, or so-called stepper, there is provided a laser step alignment (LSA) system of through-the-lens (TTL) method as disclosed in the U.S. Pat. No. 4,677,301 for exactly aligning the projected image of the reticle pattern with the circuit patterns formed in a matrix form on the wafer. Such LSA system has a wide mark detecting range (search range), and is capable of high speed measurement for alignment. For this reason, the enhancement global alignment (LGA) by such LSA system, as disclosed in the U.S. Pat. No. 4,710,026, is principally used for the alignment in the stepper. However, because of the recent progress in the miniturization of circuit patterns, such conventional alignment system has become unable to obtain sufficient precision of measurement for alignment.
Consequently, as an alignment (mark detection) method for obtaining a still higher precision, there has been proposed a method, as disclosed in the U.S. Pat. No. 4,636,077, of projecting static interference fringes on a diffraction grating pattern formed on the wafer, moving said interference fringes relative to the diffraction grating pattern and aligning the wafer based on the intensity change of the interference light generated from said diffraction grating pattern. This method is based on the technology of the U.S. Pat. No. 3,726,595 for measurement of displacement utilizing a fact that the relative positional change between the diffraction grating pattern and the interference fringes unitarily corresponds to the intensity change (sinusoidal level change) of the interference light.
As another method for measuring displacement utilizing the interference fringes and a diffraction grating pattern, the U.S. Pat. No. 4,710,026 discloses a technology of moving interference fringes at a high speed in the direction of pitch of a diffraction grating pattern, and detecting the positional aberration of the diffraction grating pattern (within .+-.1/4 of the grating pitch or a multiple thereof) from the phase difference between an AC photoelectric signal obtained from the interference beat light coming from said diffraction grating pattern and an AC reference signal corresponding to the moving speed of said interference fringes. This method is also called heterodyne method, because of the utilization of a light beat signal, while the aforementioned method utilizing the static interference fringes is called homodyne method. In said heterodyne method, the phase difference between two AC signals of a same frequency (measured signal and reference signal) corresponds unitarily to the displacement of the diffraction grating pattern, and can be measured with an extremely high resolving power even with a simple phase meter.
As an example, let us consider a case in which the diffraction grating pattern on the wafer has a pitch P of 2 .mu.m (lines and spaces of a width of 1 .mu.m each), and the phase meter has a resolving power .DELTA..theta. of .+-.0.5.degree.. Under certain conditions, 1/4 of the grating pitch P in the heterodyne method corresponds to a phase difference .+-.180.degree., so that the resolving power .DELTA.X of the measurement of displacement is determined as follows: EQU .DELTA.X/.DELTA..theta.=(P/4)/180
Consequently, under the above-mentioned conditions, the resolving power .DELTA.X is as high as 0.0014 .mu.m. Also stability of measurement is high because the phase difference is obtained from the average of signal waves over a certain period (in the order of milliseconds). Furthermore this method is not affected strongly by the fluctuation of the signal level as in the homodyne method, since the alignment is not dependent on the change in signal level but on the phase change.
However, as the information on positional aberration obtained in the conventional homodyne or heterodyne method is repeated at every 1/2 of the pitch P of the diffraction grating pattern, there will result an alignment error equal to a multiple of P/2 if the pre-alignment of the pattern is insufficient. Consequently a pre-alignment is required for the diffraction grating pattern within a precision of .+-.0.5 .mu.m for a pitch P=2 .mu.m. In such mark detecting method, the pitch P of the diffraction grating pattern will become finer in the future, and in that case especially, conventional mechanical prealignment will not be adequate.